1. Field of the Invention
The present invention relates to an improvement in a ball spline bearing and, more particularly, to a cheap and rigid bearing having a small slide resistance which can be easily worked with a high degree of accuracy.
2. Description of the Prior Art
As prior arts of this kind of bearing, the bearing shown in FIGS. 1A and 1B (Japanese Patent Unexamined Publication No. 39937/1972) and the bearing shown in FIGS. 2A and 2B (Japanese Patent Examined Publication No. 7601/1970) are well known. The ball spline bearing of FIGS. 1A and 1B is constituted in a manner such that: shallow grooves and deep grooves are alternately formed in the inner surface of the outer cylinder; track grooves are formed on both side surfaces of the shallow groove; balls are arranged in the track grooves; and a shaft having coincident protuberances is inserted into the space. According to this bearing, the shape of the inner surface of the outer cylinder is complicated, so that there are drawbacks such that the working accuracies regarding the flatness of the track groove, the parallel degree among the respective tracks, and the like are low, and the lives of the balls in the load range often become short depending on their locations since the load distribution is not uniform. Further, since the protuberances of the shaft are protruded so as to be inserted into the right and left load balls, the geometrical moment of inertia of the axis is small and the problem regarding the rigidity of the shaft accordingly occurs.
Further, according to the ball spline bearing of FIGS. 2A and 2B, the load balls are moved outwardly in their radial directions and reach the return passage; therefore, there are drawbacks such that the outer diameter of the bearing becomes too large and this bearing cannot be miniaturized.
As described above, the conventional bearings have the drawbacks, respectively. A small-sized bearing having a high rigidity in which the accuracies of the track grooves are high is strongly demanded.